This application claims priority to German Application No. 100 01 650.2 flied on Jan. 17, 2000 and International Application No. PCT/EP01/00498 filed on Jan. 17, 2001, the entire contents of which are incorporated herein by reference.
The invention relates to a high-strength, age-hardenable, corrosion-resistant maraging type spring steel.
Alloys which are fully martensitic in the solution-annealed state are used which are age-hardenable by heat treatment. These alloys exhibit good isotropic deformability prior to age-hardening. After age-hardening, these alloys display very high strength, hardness, fatigue strength under reversed bending stress, and relaxation resistance less than 300xc2x0 C. Such alloys are known, for example, from European Patent Application 0 773 307 A1 and from Japanese Patent Application A-49 119 814.
These maraging type spring steels are distinguished from metastable austenitic or semi-austenitic steels primarily by their martensite temperature. For metastable austenitic or semi-austenitic spring steels, the martensite temperature is approximately at or below room temperature. Such metastable austenitic or semi-austenitic steels are known from European Patent Application 0 210 035 A1, for example.
The aforementioned steels require increased cold forming in order to form strain-induced martensite. They have the distinct disadvantage that in the production of wires and strips, the ductility is severely reduced by the increased cold forming before the actual age-hardening. In particular for the production of strips, a so-called deformation texture forms which prevents isotropic deformability. Here and in the following discussion, xe2x80x9cisotropic deformabilityxe2x80x9d is understood to mean that the deformability is comparable both parallel and perpendicular to the direction of rolling.
However, in the use of spring steels for spring elements, which must fulfill a plurality of functions simultaneously, such isotropic deformability is absolutely essential.
A high-strength, corrosion-resistant spring steel is known from the previously mentioned Japanese Patent Application A-49 119 814 which comprises nickel and chromium in the range (2.5; 14), (10.2; 14), (7.3; 18), and (2.5; 18) on the (nickel; chromium) weight-% diagram, with the remainder comprising iron. For heat treatment, Japanese Patent Application A-49 119 815 recommends at least one of the elements molybdenum, titanium, copper, tungsten, or zircon in a total proportion of less than 0.5% by weight. For age-hardening, a beryllium content greater than 0.3% by weight is recommended. It has been shown that when a beryllium content greater than 0.3% by weight is used, even when the titanium additives of the teaching are also used, the alloy could not be heat treated.
A high-strength, corrosion-resistant spring steel is known from the previously mentioned European Patent Application 773 307 A1 which comprises 6 to 9% by weight nickel, 11 to 15% by weight chromium, 0 to 6% by weight copper and cobalt, and a combination of molybdenum+xc2xd tungsten in the range of 0.5 to 6% by weight and beryllium in the range of 0.1 to 0.5% by weight. However, in this case it has been shown that this material is not effective in production because in some cases it is dual-phase; that is, in addition to martensite it also contains high proportions of ferrite. However, this proportion of ferrite results in undesired mechanical properties. On the one hand, proportions of ferrite in the aforementioned compositions can rise as high as 60%, resulting in reduced lattice distortion and thus loss of hardness before and after age-hardening. On the other hand, during heat treating in the unfavorable temperature range between age-hardening and solution annealing, the ferrite can decompose into a brittle theta phase which upon cooling converts to martensite. This decomposition results in greatly decreased ductility.
Furthermore, in the aforementioned compositions the martensite temperature in some cases is too low, for example, xe2x88x9240xc2x0 C. And, even for compositions with martensite temperatures that under normal conditions are approximately 100xc2x0 C., in some cases it is possible that the austenite is not completely converted to martensite. The temperature and duration of annealing in addition to the quenching speed have been found to be critical processing parameters. This results in sharp declines in hardness in the age-hardened state and marked fluctuations in quality during production.
In addition, spring alloys are known from Swiss Patent 320 815 which can comprise up to 25% by weight chromium and up to 20% by weight nickel. The alloys described therein may be austensitic as well as ferritic or martensitic, and may also be present in combinations of austensite, ferrite, and martensite. As a rule, with the wide alloy windows described therein, the mechanical properties, in particular a good, reproducible isotropic deformability, cannot be assured.
Furthermore, an austensitic superalloy based on cobalt-nickel is known from Swiss Patent 265 255. The cobalt-nickel-based alloy described therein is provided with hardening additives of beryllium and/or titanium and/or carbon in quantities of up to 5% by weight. The alloys described therein are austensitic, with the result that fairly high beryllium concentrations are necessary to age-harden them since the solubility of beryllium in an austensitic structure is relatively high.
In addition, a method for adjusting textures in ferritic alloys is known from German Laid-Open Patent Specification AS 1 186 889.